[2] At the outbreak of the Civil War in 1861, Gates enlisted in the Confederate Army, starting as a captain in the Missouri State Guard under the command of General Sterling Price, and was later promoted to colonel of the 1st Missouri Cavalry Regiment.

He commanded his regiment at the Battle of Pea Ridge, Arkansas in March 1862, during the Siege of Corinth, Mississippi, and at the Battles of Iuka, Second Corinth, Champion Hill, Big Black River Bridge and at the Siege of Vicksburg, Mississippi.

On April 9, 1865, the same day Confederate General Robert E. Lee surrendered to Union General Ulysses S. Grant, Colonel Gates was engaged in one of the last battles of the Civil War, the Battle of Fort Blakeley in Alabama.

In 1874, he was elected as Sheriff of Buchanan County, serving in that post until 1877.

Following his tenure as State Treasurer, he served as United States Marshal for the Western District of Missouri under President Grover Cleveland, and was engaged in the transfer and bus business in St. Joseph, Missouri until his death at the age of 87 years old.